The Science of Sex Chromosomes

One of the most common attacks on trans people that I see can be summed up as, “But, CHROMOSOMES!!!”. We know, after all, that our chromosomes are in every cell of our bodies. We are, allegedly, indelibly either male or female. That, we are told, cannot be changed. People advancing this view always claim to be arguing scientifically. They know very little about science. Part of my job, in doing trans awareness training, is to disabuse them of their foolishness. When I discovered Sex Itself by Sarah S Richardson I knew I had to read it. I am very glad that I did.

Sex Itself is essentially a history of the science of sex chromosomes. It is a fairly short history, starting in the early 20th Century, but is none the less fascinating if you have enough of a science background to not be put off by the language, which gets very technical at times. It is an object lesson in how cultural attitudes inform scientific research. Even the term, “sex chromosomes” is controversial. I use it here primarily because I want people with a poor understanding of the issues to read this post, and I know how search engines work.

Let’s start with a simple statement (emphasis mine):

Biologists have never been under the illusion that genes and chromosomes are all there is to the biology of sex. […] Today, academic sexologists typically distinguish between chromosomal sex, gonadal sex, hormonal sex, genital sex and sexual identity. Some would add sexual preference, gender identity, morphological sex, fertility and even brain sex to this list.

So yes, it has never been the case that science thought that sex was determined solely by chromosomes. Sorry, TERFs*.

Real science is complicated, messy. We still don’t really understand all of the biological pathways that result in the various facets of sex. What we can say is that X = female, Y = male is nonsense.

To start with, not all animals use the XX/XY system of chromosomes. Birds, for example, have a very different, and much more complicated system. And yet birds occur in male, female and intersex forms. There are species in which the form with XY chromosomes would normally be regarded as female (i.e. produces eggs rather than sperm). There is even a mammal species, the mole vole, that doesn’t have Y chromosomes. They have XX and XO variants, but the XOs still have recognizably male behavior and play a male role in reproduction.

Even in humans, “correct” configuration of the sex chromosomes is neither necessary nor sufficient to produce a body of a particular sex. There is at least one well documented case of a person with XY chromosomes getting pregnant and giving birth (to a daughter with XY chromosomes). And differentiation of ovaries and testes in the embryo is dependent on two genes on other chromosomes as well as the XX/XY pair.

Despite all of this obvious science, the book chronicles endless attempts by scientists to find a magic switch that is “sex itself”, the ultimate determinant of human nature; and to prove once and for all that men are from Mars and women from Venus. One of the most ridiculous examples of this is the 2005 Nature paper by Carrell & Willard which claimed that there is as much, if not more, difference between the genome of a human male and a human female, than there is between that of a human and a chimpanzee. It doesn’t take much critical thinking to find the flaws in that, but Richardson, good scientist that she is, goes in detail into the different methods used to calculate “difference” in the human-male/human-female case, and in the human/chimpanzee case, to show that the comparison is invalid. And she makes the philosophical point that a genome is a property of a species, not of a sub-form of a species that is incapable of independent reproduction.

Another example is the ongoing debate between David Page and Jennifer Graves over the status of the Y chromosome. Page maintains that the Y is a noble beast and source of all that is great and good in humans; while Graves maintains that it is a wimpy runt with no great purpose nor any evolutionary future. I exaggerate a little for effect, but both scientists openly use sex war rhetoric in their debates so they really can’t complain. The controversy has found its way into the media, and into popular culture. Richardson cites Gwyneth Jones’s Life and Brian Vaughan & Pia Guerra’s Y: The Last Man as examples.

My own preconceptions were not immune from Richardson’s intellectual scalpel. The easiest way to explain the results of Androgen Insensitivity Syndrome (the intersex condition that gives rise to things like the aforementioned XY pregnancy) is that human bodies are default female, and need a variety of processes, triggered by the Y chromosome, to make a male. I now know that this isn’t strictly true. An embryo will not develop as fully female without other active processes.

This issue shows how complex the cultural issues surrounding gender science are. I used to think that I was being proudly feminist in stating that being female was the default state of mankind, and that being male was some sort of weird mutation. I now find that I am a dupe of the Patriarchy for believing that being female is a passive state waiting to be conquered and controlled by the masterful Y chromosome. I am suitably chastised.

Another area where I have had to modify my own understanding is the relationship between Klinefelter syndrome and gender identity. Klinefelter is a condition where the body has XXY chromosomes. When I was young, this was thought to be potentially diagnostic of trans women. I remember being distraught when my chromosome test came back as XY, because an XXY result would have fast-tracked me through the medical system. However, from what Richardson says it seems that trans women are not over-represented in the population of people who exhibit Klinefelter, and consequently the condition is neither diagnostic of, nor a potential explanation of, our gender identity.

This brings me to the most spectacular example of sex science nonsense in the book, and a possible explanation for the beliefs of the TERFs. Klinefelter is by no means the only condition in which a body’s chromosome mix is neither XX nor XY. One of many others is the so-called “super-male”, XYY. Back in 1965 Patricia Jacobs, a brilliant geneticist who discovered the biological cause of Klinefelter when she was just 21, published a study in Nature showing that inmates with XYY were over-represented in a high security psychiatric institution.

Sadly Jacobs wasn’t near as good at social science as she was at analyzing genes. There are all sorts of flaws with the study, including the later discover that XXY conditions were similarly over-represented. But before you could say “radical feminism”, the idea that a Y chromosome was an indicator of criminal violence, and two Ys doubly so, was all over popular culture. There was even a series of books, and later a TV series, called The XYY Man.

The idea that XYY was an indicator of a violently criminal nature has long since been debunked, but the idea that a Y chromosome is the seat of violence is still very much current among radical feminists and is often cited as “proof” of why trans women should not be allowed into female spaces. Personally I think that if there is any culprit then it would be testosterone, and the Nazi attempts to produce a super-soldier serum (based on research stolen from Magnus Hirschfeld’s sex clinic) would seem to back me up. Doubtless the TERFs would claim that it only takes one drop of testosterone in utero to turn a human into a violent psychopath, so trans women still can’t be trusted.

And yes, I did use the phrase “one drop” deliberately there. Many of the flawed scientific studies that Richardson describes in the book reek of eugenics, and a book I now want to read is Stephen Jay Gould’s Mismeasure of Man, which chronicles scientific attempts to prove white supremacy.

Science, as I noted above, is complicated, and the interaction of science with society doubly so. I totally understand the need to examine how medical conditions differ between males and females (and indeed between people of different ethnic groups). As someone whose body is now physically intersex (thanks to medical intervention) I have a vested interest in such things. But the obsession that humans have with categorizing things in binaries, and with using popular misunderstandings of science as a crutch for bigotry, makes all such work very dangerous. I am very grateful to Sarah Richardson for shining a bright light on the murky issue of chromosomal sex. Hopefully I can do her work justice in future training courses.

4 thoughts on “The Science of Sex Chromosomes”

I am deeply suspicious of any popular science book with “brain sex” in the title. A quick Google, and checking Cordelia Fine, suggests that it is just the sort of thing that Richardson is complaining about.

Thanks for recommending this book, Cheryl. It looks really interesting. I teach biology classes for pre-nursing students and for non-science majors, and our textbooks are rife with oversimplifications, if not outright errors. I’ve been wanting to find an overview of the issue this book discusses for some time.

I just finished reading Sex Itself a couple of weeks ago. I learned TONS of cool stuff from it, particularly the fact that lots of the genes for sex determination seem to be on the autosomes not the ‘sex’ chromosomes.

I’d also recommend Sexual Selections: What We Can & Can’t Learn About Sex From Animals by Marlene Zuk. She’s a biologist and a feminist but is happy to have a go at both camps (and at misogynists) for holding up animal behaviour as a model for ‘proper’ human behaviour. Her other book (Paleofantasy: What Evolution Really Tells Us About Diet & How We Live is also worth a read).

Meanwhile on TERFs… I recalled being taught that oestrogens are manufactured out of testosterone and other ‘male’ hormones. Since this was back in the 80s I thought I perhaps was wrong, but no – here is a link to a (complicated) diagram. Down in the bottom right it shows testosterone as the precursor for oestrodiol! So the TERFs don’t know any biochemistry as well as not knowing any genetics! 🙂https://upload.wikimedia.org/wikipedia/commons/1/13/Steroidogenesis.svg